Remove calls to throw helpers for validating input

[oliverbooth]

Summary

Replace all methods containing the following call:

ArgumentNullException.ThrowIfNull(...);

with the following:

if (... is null)
{
    throw new ArgumentNullException(nameof(...));
}

Rationale

On a personal level, I find it inconvenient that Rider fails to generate the <exception> node in XMLDoc using a throw helper, since there's no explicit throw statement in any immediate code path.

On a more important level, however, throw helpers obscure the call stack and more vitally introduce a problem in test coverage. Since a single method call instruction is covered in all cases, coverage can falsely be reported as 100% despite no unit tests existing for a null check. A branch with an explicit throw, however, will guarantee that tests reach true 100% coverage by adding in tests cases for null input, without exception*.

* Pun intended.